Line a 9-inch square pan with foil, with ends of foil extending over sides; spray with cooking spray.,Cook sugar, milk, butter and salt in large saucepan on medium heat until butter is melted and sugar is completely dissolved, stirring constantly.,Bring to boil, without stirring; cook, 8 min.,or until candy thermometer reaches 238 degrees F or soft-ball stage (when 1/2 tsp.,syrup dropped into very cold water forms soft ball that flattens when removed from water).,Remove from heat.,Add chocolate; stir until melted.,Beat with mixer 6 to 8 min.,or until fudge is thickened and no longer glossy.,Pour into prepared pan.,______,Sprinkle with nuts and marshmallows; press gently into fudge with back of spoon.,Cool completely.,Cool completely.,Use foil handles to lift fudge from pan before cutting to serve.